Photoelectric exposure meter having a reflector facing the photocell



April 1l, 1950 RlszDoRFER 2,503,768

PHOTOELECTRIC EXPOSURE METERS HAVING A REFLECTOR FACING THE PHoTocELL Original Filed Sept. 25, 1940 4 Sheetssheet l gll Illlllllllllllllllllllllll Q.. RzszooRFE/e.

April 1l, 1950 v L. RlszDoRFER 2,503,768

PHoToELEcTRIc EXPOSURE METERS HAVING A REELEcToR EACING THE PHoTocELL Original Filed Sept. 23, 1940 4 *Sheets-Sheet 2 L'. Riszoo/arm.

Apri] 11, 1950 RlszDoRFl-:R 2,503,768

PHOTOELECTRI EXPOSURE METERS HAVING A REFLECTOR FACING THE PHOTQCELL.

Original Filed Sept. 25, 1940 4 Sheets-Sheet 5 lwucm/to'o l. Riszpanrm.

Aprl 11, 1950 L. RISZDORFER 2,503,768

PHOTOELECTRIC EXPOSURE METERS HAVING A REELEoToR EACING THE PHoTocELL Original Filed Sept. 25, 1940 4 Sheets-Sheet 4 rwc/niE/b "L, Rszno/EFER.

Patented Apr. 11, 1950 PHOTOELECTRIC EXPOSURE METER HAV- ING A REFLECTOR FACING THE PHOTO- CELL Laszlo Riszdorfer, Budapest, Hungary; vested in the Attorney General of the United States Original application September 23, 1940, Serial 357,992. Divided and this application April 30, 1945, Serial No. 591,105. In Hungary August 26, 1939 2 Claims.

The magnitude of the langle of incidence under which the light falls on the photo-cells of the exposure meters of the types known up to now, as well as on those of the semi-automatic or automatic light control devices of the photocell type employed in connection with photographic cameras, amounts to about 60 to 80 degrees; in the case of some devices this angle is even greater. This is due to the fact that in case of a smaller image angle, the quantity of light transmitted to the cell will not be suiiicient to satisfy practical requirements. Owing to the use of an image angle of so great magnitude, it is an average quantity of light that will reach the photo-cell, so that the photo-cell will practically never be operated by the light of a definite point, but by the light reaching the photo-cell from the sky, from the ground and from various other objects jointly, i. e. by the mean gure of the quantities of light reaching the cell from all these different quarters.

In practice this gives rise to different defects. Notably, in the case of exposure meters of this type of design, or in the case of photographic cameras equipped with a semi-automatic or completely automatic device of the photo-cell type operated by means of a photo-cell having such an image angle, the measurement of illumination effected on the basis of the average quantity of light reaching the cell, and accordingly 'L i. e. behind the person taking the photograph,

that is to say if the taking of the photographs is effected with so-called flat illumination. If, however, the taking of the photograph or the measurement of the necessary amount of exposure takes place in side or opposed light, this measurement of the necessary amount of exposure according to average quantity of light, and, accordingly, the exposure effected by means of a camera, equipped with a device of the kind referred to will be incorrect. This is due to the fact that it is always on the basis of adjustment for the parts in shadow that the measurement of the necessary amount of exposure and the exposure itself, respectively, have to be eifected.

This means in practice that the actual amount of exposure which has to be employed will in the case of side light exceed the figures of exposure obtained on the basis of the average quantity of light by about 30 to 50%, in the case of opposed light by 50 to 100% or possibly even by more.

Owing to the greatest latitude offered bythe black and white emulsion substancee this fault will not manifest itself in any striking manner in the case of the devices for the measurement of the necessary amount of exposure or for the control of light which are operated on the basis of an average quantity of light of the types employed up to now, but it will present itself all the more strikingly in the case of colour photography, for which, accordingly, the devices of this kind can only be used in the case of the so-called flat exposure of the subject. Notably, in the case of side or opposed light it is necessary, for the purposes of measurement, either to approach r these devices quite closely to the shadow portions of the object to be photographed, which means a great deal of inconvenience and loss of time, or to act according to a judgment of conditions of illumination based on an estimate, which method may give rise to very many errors, and degrades the exposure meter of the photocell type to the position of an instrument the use of which must be based on personal judgment.

Accordingly, only imperfect results have been secured from the use of known devices based solely on the so-called total illumination value or not depending primarily on the contrasts of i1- lumination of the object to be photographed.

The devices according to the invention eliminate all these drawbacks by the arrangement according to which to the device of the photo-cell type which up to now has been measuring exclusively the mean ligure of illumination-which device may be not only an exposure meter but also a semi-automatic or completely automatic light control devicethere is added an apparatus automatically compensating the said device, thereby enabling that the device should take into account not only the mean degree of illumination, but also the contrasts of light or contrasts of illumination of the object to be photographed, and also its colour values.

Notably, the contrasts of illumination of the object depend in the first place on the fact from what angle, based on the position of the photographer, the object to be photographed is illuminated by the source of light from which light falls on it. Notably, if the light falls on the object from behind the photographer, the data of measurement based on the average quantity of light do not require any correction. If, on the other hand, it is from the side that the light of the source of light, e. g. the light of sun, falls on the object, a correction of the data will already become necessary, whereas if it is from behind the object to be photographed that the light falls on the latter, the correction will have to be even greater.

It is, accordingly, evident, that the greater the intensity of the light to be dealt with, e. g. intense sunlight, and the less it is from the front that the rays of this source of light are illuminating the object to be photographed, the greater will be the correction required.

It is this correction which is provided automatically by the various kinds of apparatus according to the invention, this being done in such a manner that the said apparatus will automatically correct the exposure meter so as to take these points of view into account.

The simplest way to achieve this is by employing a type of apparatus in which the operation of the device for the measurement of the necessary amount of exposure or for light control is influenced in a suitable manner by another device of the photo-cell type, or possibly without the employment of a special photo-cell, either by optical or by electrical, or by mechanical or by any other known means.

The energy possessed by these compensating devices of the photo-cell type will, of course, only have to be sufficient for suitably correcting the energy of the exposure meter proper. Another fundamental condition of successful operation is that the device according to the invention for the measurement of the necessary amount of exposure or for light control will have to be calibrated in such a manner that in those cases when no correcting effect whatever has to be displayed by the compensating device, as for instance in case of measuring the necessary amount of exposure of an object illuminated by entirely iiat illumination, such an exposure meter should indicate gures exactly equal to those indicated by another device for the measurement of the necessary amount of exposure or for light control not equipped with any compensating apparatus according to the invention.

It is a matter of course that all devices according to the invention for the measurement of exposure or for light control can be employed not only for all kinds of photometric measurements, but also for correcting the operation of semiautomatic or completely automatic devices of the photo-cell type employed in connection with photographic cameras, but for the sake of greater simplicity we shall, in the remaining part of this specification, pay no attention to this circumstance and refer to the devices according to the invention as if they were exposure meters only.

The plane of the surface of the compensating cell may form diierent angles with the plane of the fundamental cell.

For the sake of greater simplicity the cells marked I on the figures, on which light falls from the subject to be photographed, are called main cells, whereas the cells marked 2, which are compensating the operation of the instruments operated by the main cell by the greater or smaller quantity of luminous energy falling on them from the source of light, are called subsidiary cells.

The device according to the invention will operate in a faultless manner if the quantity of light falling from the source of light on the subsidiary cell is sufficient for causing the self-adjustment of the pointer of the instrument operated by the main cell to be inuenced in such a degree as to ensure that the said pointer should always, independently of the position of the source of light,

indicate the diaphragm or shutter gure corresponding to perfect exposure. That is to say, whether it is light coming from behind or light from the side or opposed light that is concerned, -Which opposed light may, of course, only be opposed light taken in the photographic sense, i. e. may not be, or may only in exceptional cases be, such opposed light as is situated within the image angle of the illumination meter, or, respectively, as shines also into the lens of the photographic camera-the exposure meter should always indicate that diaphragm or shutter figure which is required for the purposes of correct exposure. In other words, it should in the case of side light indicate an amount of exposure increased by about 30 to 50%, and in the case of opposed light an amount of exposure increased by to 100%, or possibly by even more, as compared to the amount of exposure indicated by an exposure meter of the types employed up to now, to the indication of which latter 30 to 100% or even more have, exactly for this reason,fto be added on the basis of an estimate, in the case of side or opposed light. The amount of correction will be different in each case and will vary from the smallest amount to the maximum, and therefore it is better to speak of limits of correction varying from 1 to 400%. In view hereof it is necessary that the quantity of energy received from the source of light by the subsidiary cell should be all the greater, the greater the difference by which the diaphragm or shutter figure indicated by the exposure meter not equipped with the apparatus according to the invention is smaller than the iigure required from the point of View of correct exposure.

This purpose can be achieved by ensuring that, differently from what is the case with the main cell adiusted on the obiect to be photographed, the image angle of which cell is relatively low, the subsidiary cell should receive light practically only from the source of light and not from the obiect, although some light may reach it from the latter also. If the image angle of the subsidiary cell is very large, amounting e. g. to degrees, and if at the same time its plane surface extends during measurement in a direction sloping upward, this is already suiiicient for ensuring that if the light of the sun is approximately opposite to it, which means that the rays are falling on its surface approximately perpendicularly, the subsidiary cell should exert the maximum of compensating effect. On the other hand, if in the case of side light the light reaches the said plane slightly from the side, the subsidiary cell will in that case also exert its compensating effect, but in a substantially smaller extent, and this is indeed what is necessary, because in the case of photographs taken with side light it is not necessary to increase the exposure, as compared to the figures indicated by the instruments of the photo-cell type employed up to now, by so much as would be necessary in those cases when the sun more or less faces the photographer.

In order to ensure the perfect operation of the device according to the invention, it is, however, not only required that the mutual proportion of the angles of the two cells should be convenient, but provision has also to be made for the angle of the subsidiary cell being sufciently large, or for ensuring by means of reecting devices that the desired quantity of light should reach the subsidiary cell in all cases, and further for the surface area and the energy of the two cells, as well as all auxiliary devices serving for rendering the mutual eliects of the two cells on each other perfect, as for instance all instruments, covering plates, etc., being of such a kind as to render the compensation perfect in all cases. It is easy to take all these factors into account empirically, and to ensure hereby that the device according to the invention should in all cases operate in a perfect manner.

Calibration is facilitated by the fact that it is necessary that in the case of at lighting, i. e. in case the source of light illuminates the object to be photographed from behind the photographer, and in case there is hardly any shadow portion on the object to be. photographed, the cxposure meter according to the invention should indicate figures equal to those indicated by the exposure meters of the types employed up to now operating on the basis of an average quantity of light. v

The exposure meter equipped with an automatic compensating device according to the invention should be amplied by the employement of a colour lter arranged in front of the cell or cells, which filter will in case the photo-cell is less sensitive for the so-called photographically dark colours, correct the measurement of light in a suitable manner from the point of View of such colours, in the first place of green, brown and red colours, situated on the area of the object to be photographed. The practical arrangement to be employed may be of very many kinds, because, as already mentioned above, it is not only in connection with exposure meters of the photo-cell type, but also in connection with all kinds of semi-automatic and completely automatic devices of the photo-cell type that the apparatus according to the invention can be employed. Thus, for instance, also in connection with the devices described in the British Patents Nos. 395,808 and 483,775' and in the French Patent No. 725,313.

Most of the various practical arrangements are illustrated on'the figures enumerated below, in

which- Figure 1 is a diagrammatic sectional View of an exposure meter according to the invention,

comprising main and compensating photoelectric cells for operating the exposure measurement device;

Figure 2 is an elevation of the structure shown in Figure l;

Figure 3 is a diagrammatic sectional view illustrating a modification of Figure 1, and ern- Figure 8 is a view similar to Figure 7, but with a different indicating device associated therewith;

Figures 9 and 10 illustrate an arrangement in which a specially constructed indicating device is operated by the Ysubsidiary photo-cell current 75 main photo-cell I.

small magnitude.

the two cells operate in association with only a single-coil instrument.

In the arrangement according to Figs. l and 2, l denotes the fundamental photo-cell and 2 the compensating photo-cell; for the sake of greater simplicity these will be called in this specincation the main and the subsidiary photo-cell, respectively. The device for the measurement of eX- posure or for light control is operated by the main photo-cell l, and the operation of the latter is decreased more or less by the subsidiary photocell 2.

It is a preferably possible arrangement to mount both photo-cells on a single base-plate. Their surfaces may be situated in the same plane, or in different planes. cell is situated e. g. in a vertical plane, the surface of the subsidiary photo-cell may be facing obliquely upwards. It is, however, also possible just as well to imagine it arranged in any other plane, particularly if use is made of certain reecting devices. It is also possible to employ more than one subsidiary photo-cell, similarly it is also possible to employ subsidiary photo- -gj cells having a convex or concave surface instead of a plane surface, and, similarly, the reflecting surfaces employed may also be of any desired shape.

According to the arrangement shown by way of example on Fig. 1 the surface area of the main photo-cell l is substantially greater than that of the subsidiary photo-cell 2. In iront of the subsidiary photo-cell 2 a light lter may also be provided which will allow more or less light to reach the cell from different directions, in accordance with practical necessity.

Of course, there is nothing to prevent the quantity of the light reaching the photo-cell either in case of the main or of the subsidiary photo-cell, the mutual angle formed by their planes, or the position of the reflecting surfaces being varied at any time by the photographer by means of any kinds of known devices.

For the purpose of projecting the side lights on the subsidiary photo-cell 2, it is also possible to employ devices enabling light to fall on the subsidiary cells even in case of side light of very This may be achieved by two subsidiary cells being arranged in the form of a wedge.

The arrangement according to Figs. 1 and 2 may also be varied so as to mount or construct the cells I and 2 so as to be situated in the same plane, possibly mounted on a common rear electrode plate.

Arrangements are also possible by means of which the side light also may be vprojected in a certain extent on the subsidiary cell 2 by means of mirrors or reflecting surfaces, which may be of any desired shape and of any desired curvature and of different strength of reflection.

Figs. 3 and 4 show an arrangement in which the subsidiary cell 2 on which the light is projected by the mirror l is mounted behind the This mirror or reflecting In case the main photo-- surface may also be of concave shape or it may comprise a plurality of reecting surfaces arranged at different angles.

The various kinds of apparatus of the photocell type described in what precedes possess a function based on such optical, electrical or mechanical principles to be described in detail below, which will ensure that in the case of side or opposed light the time of exposure measured by the main cell, and, correspondingly the shutter aperture of the object lens should be increased. The same eiect may also be achieved by Calibrating the time of exposure measured by the main cell in the case of side and opposed light at the figure increased to the necessary` correct gure, or correspondingly by adjusting the operation of photographic cameras equipped with semi-automatic or completely automatic devices in a suitable manner, so that if the source of light is behind the operator, this light coming from behind should be projected on the fundamental or main cell itself, or possibly on a special subsidiary cell, and the exposure meter should in conformity with the greater eect of illumination of the said light measure a shorter time of exposure, or in the case of photographic cameras equipped with the devices in question adjust a smaller aperture of the object lens shutter.

In the case of this arrangement the photographer should take care to ensure that the source of light behind him should not be covered during the handling of the apparatus by any part of the apparatus.

Such an arrangement is illustrated on Figs. and 6.

On Fig. 5 the photo-cell 8 is greater than the image angle limiting device 9, and extends above the latter. It is on this upper part of the cell 8 that the light coming from behind is projected by the mirroring or diffusely reflecting surface IG. The upper part of the reflecting surface Ill may also be constructed so as to be semi-transparent in different gradations.

On Fig. 6 the partly transparent plane surface or screen of curved surface II is arranged in front of the cell 8 equipped with the image-angle limiting device 9, in such a manner as to ensure that the light coming from behind should be refiected by it on the cell in the desired degree. In these two last-named arrangements the employnient of a special subsidiary cell has accordingly been dispensed with. Both arrangements can preferably be disposed in such a manner that the light coming from behind should be reflected on the photo-cell 8 not from above, but from the side.

Various kinds of circuit arrangements may be employed for the devices described above. The most simple method is to connect the two photocells directly into a common circuit with the Deprez instrument. This can be performed in two different ways: (l) The two cells are connected in series in opposition to each other, i. e. the identical electrodes (e. g. the positive electrodes) are connected with each other, whilst the other two (in this case the negative) electrodes are connected to the terminals of the instrument. Such an arrangement is illustrated on Figs. l and 2. (2) The two cells are connected in parallel in opposition to each other, i. e. their opposite poles are connected with each other, and it is to these poles that the instrument is connected. Such an arrangement is shown on Figs. 3 and 4. It is, however, also possible to employ a circuit larrangement in which the currents of the two photo-cells are electrically separated from each other.

Such arrangements are illustrated on Figs. 7 to 14. In the arrangement shown on Fig. 7 the main photo-cell I operates the Deprez instrument I2 in such a manner that under the action of light the pointers of both instruments should become deflected in the same sense. As the pointer I5 of the instrument I3 replaces the iixed index well-known from coinciding photometry, the deviation of the pointer I4 from the xed index will be equal to the difference of the deection of the two pointers, this is equivalent to deducting the current of the subsidiary photo-cell 2 from the current of the main photo-cell I. v

Fig. 8 shows an arrangement embodying the same principle as the arrangement shown on Fig. '7, with the diiference that here a certain scale usual in the case of photometers is constructed so as to constitute the pointer I'I of the instrunient I3, and it is relatively to this moving scale that the motion of the pointer I6 of the instrument I2 can be observed.

Figs. 9 and l0 are representing an arrangement in which the pointer of the instrument I3 operated by the current of the subsidiary photo-cell 2 is constructed as a light-weight plate I8, which, in proportion to the quantity of light reaching the subsidiary photo-cell 2, covers a smaller or greater part of the main photo-cell I. The unilateral torque of the plate I8 is balanced by the plate I9, which latter at the same time also prevents any possibility of the main cell I being over-compensated or over-covered by the subsidiary photo-cell 2 under the direct effect of very strong opposed light, because with a so great deflection of the instrument I3 the plate I9 will already cover a part of the subsidiary cell 2, and will thereby weaken the current of the latter. In order to ensure the correct operation of the apparatus, the plates I8 and I9 may be constructed so as to possess a profile of advantageous shape. The current of the main photo-cell I may then be used in any known manner also for the purposes of the automatic exposure meter.

Fig. 11 represents an arrangement in which the two photo-cells I and 2 are acting on an instrument comprising a double oscillating coil 2|). One end of the two coils may also be made common, and the current may be conducted on it by body-grounding it through the metal casing of the apparatus, so as to enable thereby the number of the flexible outgoing leads of the coils to be diminished from four to three or two, respec'rively. One cell acts on one of the coils, and the other cell on the other coil, and in con sequence hereof, the pointer will become displaced in proportion to the difference of the properly adjusted ampere turns of the two cells.

Figs. l2 and 13 represent an arrangement in which the current of the main photo-cell I acts on the instrument 22, the deflection of the pointer 23 of which may be observed in front ofthe scale 24. The correct image angle of the light reaching the main cell I is assured by the imageangle limiting device 25.

The subsidiary photo-cell 2 is arranged, without any image-angle limiting device, on the front upper part of the apparatus, in such a manner that the light from behind does not act on it all, whils't side light already exerts a certain action on it and opposed light acts on it most strongly.' The current of this subsidiary cell acts on the instrument 26, on the axle 2'I of which the nontransparent plate 28 of very light weight is xed.

. cutanee This plate 28 is adjusted in such a manner that the greater the quantity of side light or opposed light available, the greater will be the proportion of light screened from reaching the main photocell I by this plate 28 under the action of the subsidiary cell 2, and the greater will be the extent in which the deection of the pointer 23 of the instrument 22 will be diminished. This diminution is of such extent as to ensure that the greater exposure which can thus be rea-d on the scale 24 should be exactly correct, so as to correspond to the amount of side or opposed light concerned.

In View of the fact that the covering effectproduced by the plate 28 is not proportionate tothe deection of the instrument 26 (as it actually follows a sine law), the terminals of the instrument 26 should preferably be shaped in such a manner as to cause the plate 28 to produce` an amount of cover proportionate to the current of the subsidiary cell 2.

Fig. 14 represents an arrangement possessing the advantage of requiring a single-coil instrument only. Notably here a certain voltage drop is produced on the resistance 29 by the current of. the main cell l, Whereas the subsidiary cell 2 produces a certain voltage drop on the resistance 30@ The instrument 3| measures the difference of the two voltage drops, which in accordance with the principle concerned is all the smaller, the greater the quantity of light received by the subsidiary I, cell 2 as compared to that received by the main! cell I.

Accordingly, the double cover illustrated on Figs. 9 and 10 is not absolutely necessary, as appears also from the arrangement represented on Figs. 12 and 13.

Although Figs. 1 to 14 are all representing exposure meters, it is nevertheless directly possible that the pointers operated in these devices should act the rle of the pointer employed on a photographic camera equipped with a semi-.automatic device, as described e. g. in the Hungarian Patent No. 109,132, and indeed also the rle of a pointer employed on a photographic camera equipped with a completely automatic device, as, for in- 10 stance, that of the pointer 3 shown on Fig. 1 of the Hungarian Patent No. 120,431.

The present application is divided out of the pending application of Laszlo Riszdorfer, Serial No. 357,992, led September 23, 1940, for Photoelectric exposure meters, now Patent Number 2,378,433, granted June 19, 1945, and the claims herein are directed to subject-matter not specically claimed in such parent application.

What I claim is:

1. Improvement in a photo-electric exposure meter, consisting in a photo-electric cell positioned to receive light predominantly from an illuminated photographic object, a reector positioned in front of said cell and spaced therefrom, with the reecting surface facing the cell to screen `from the cell part of the light from the photographic object, said rearwardly facing refleeting surface being adapted to receive light from a direction other than that from the photographic object to the cell and to reflect said light into said cell and a measuring instrument in the circuit of said cell.

2. The photographic exposure meter as claimed in claim l wherein the reilector is partly transparent so las to permit light from the object to reach the cell through the reilector.

LASZLO RISZDORFER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,058,562 Bucky et al Oct. 27, 1936 2,129,114 Bernhardt et a1 Sept. 6, 1938 2,137,548 Strauss Nov. 22, 1938 2,145,147 Wolferg Jan. 24, 1939 2,183,217 Goldsmith Dec. 12, 1939 2,302,597 Bing N'ov. 17, 1942 2,354,544 Rath July 25, 1944 2,378,433 Riszdorfer June 19, 1945 2,382,220 Fogle Aug. 14, 1945 

